This invention relates to a method for forming a polycrystalline optical body which has higher mechanical strength than a melt-grown essentially single macrocrystal, also referred to as a macrocrystal ingot, or simply `ingot`, from which the polycrystalline optical body is derived. By "optical body" I refer to one which is essentially completely permeable to wavelengths in the ultraviolet, or visible, or infrared regions. Strong press-forged normally frangible inorganic crystals are known to be produced by hot press-forging a melt-grown macrocrystal ingot. By "normally frangible" I refer to easily breakable or readily cleavable melt-grown ingots such as those of the alkali metal halides, the alkaline earth metal halides and the like, all of which will cleave readily along crystal planes at room temperature or below, and even at elevated temperatures. Though polycrystalline bodies also shatter, ingots shatter along well-defined crystal planes if the ingots are dropped on to a hard surface. Certain inorganic crystals are not normally frangible, such as for example, silver chloride, and this invention is not directed to such crystals.
Optically integral polycrystalline laser windows, domes and lenses which are formed from rectangular blanks greater than 20 cm.times.10 cm, or discs greater than 20 cm in diameter, must have superior mechanical properties compared to those of a single crystal, without sacrificing the single crystal's optical integrity. In other words, polycrystalline laser windows and the like must be optically indistinguishable from the single crystal from which they were derived. Often, hot press-forged ingots and extrudates of ingots produce polycrystalline optical bodies having superior optical properties than those of the ingot from which they were derived.
Included among optical bodies are scintillation phosphors which generate light. Such phosphors may include dopands which are generally ionic salts. Scintillation phosphors are used for the detection of ionizing radiation in conjunction with a photomultiplier tube in devices ranging from simple scintillation counters to sophisticated camera plates for medical use in connection with the analysis of gamma radiation emanating from patients who are injected with specific active isotopes. In a large camera plate, it is essential that the plate be mechanically strong and resistant to shock. Such strength and shock resistance is provided in a polycrystalline plate derived by hot press-forging a macrocrystal ingot.
Thus, it is known in the art to hot press-forge a melt-grown macrocrystal ingot of an ionic salt as described in U.S. Pat. No. 3,933,970 to Rosette and Packer, the disclosure of which is incorporated by reference thereto as if fully set forth herein. Though the method described therein is highly effective with alkali metal halide crystals of arbitrary size, it is less effective with alkaline earth metal halides, particularly the halides of calcium, strontium and barium. It was found that a deformation ratio in excess of about 2 could only be produced with a very slow forging if the forged body was to be free of surface defects and peripheral fissures.
It was most particularly noticea that ingots of calcium fluoride could be hot press-forged as described therein, but upon cooling, the forged polycrystalline mass was susceptible to fracture, particularly near its periphery, and "veiling" due presumably to internal unrelieved stresses and undesirable dislocations. By "veiling" I refer to generally planar striations near the surface of the forged body which appear to be generated by gas bubbles or voids formed as the forged crystal cools to room temperature. Those skilled in the art are well aware that an apparenty minor surface crack or other flaw ascribed to the hot press forging process vitiates the usefulness of the press-forged optical body as a laser window, or as a camera plate, or for any application where mechanical strength is a property of critical importance.
To combat the problems of stress cracks and other flaws such as veiling, a prior art process was developed for hot forging macrocrystal in an isostatic pressure environment. In this process, a macrocrystal was forged in a hot-wall pressure vessel in both oil and gaseous environments. Oil was used effectively to about 450.degree. C., whereas gas allowed reaching 600.degree. C., the temperature limit of the pressure vessel which was used. Forging was effected at temperatures from 300.degree. C. to 600.degree. C. in pressure environments ranging from 5000 psi to as slow as 3000 psi. Crack-free crystals were obtained on a reproducible basis by forging at 300.degree. C. or higher when forging was followed by in-situ annealing at 600.degree. C. or higher.
Isostatic pressing is conventionally carried out using an isostatic press which is essentially a pressure chamber filled with a liquid or gaseous medium through which the necessary pressure is applied, essentially uniformly, to a powder held in a bag or other container which is also uniformly deformed. Isostatic pressing is commonly used to densify powder metal or ceramic `preforms` prior to sintering them. The process is particularly used to compress further, and uniformly, a difficultly compressible silicon nitride preform of compacted powder which is also bedded in silicon nitride powder, as for example disclosed in U.S. Pat. No. 4,071,372. As those skilled in the art will recognize, uniformity of application of the pressing forces to obtain continuous equalization of pressure over the body being pressed, combined with precise definition of the form which results, is the nexus of isostatic pressing. Neither continuous equalization of applied pressure nor a well-defined shape are obtained in the process of my invention, and neither is necessary or desirable.
It is also known to employ a powder of an inorganic material having high hardness and cleavage as the solid pressure transmitting medium in an isostatic pressing where it is desired to obtain an improved pressure magnitication ratio so that an ultra-high pressure may be applied to a `preform` embedded in the powder. Such a process is taught in U.S. Pat. No. 4,081,505 but there is no suggestion that the method is used for any purpose other than densification of the preform. There is no significant change of shape of the preform, and no change of any dimension of the preform relative to another of its dimensions. In this patented method the preform must be completely enveloped in powder the amount of which is arbitrary.